No need for a long complicated explanation, the North's problem came down to one word: Democrats.
Roughly half of Northerners were Democrats who until 1861 had been allies, partners and friends of Southern Democrats.
Northern Democrats had no problem with slavery and sympathized with secession.
They sure as heck didn't want to fight & die against their own party.
Properly groomed by Confederates Northern Democrats could have formed a rebellious force within the Union just as potent as emancipated slaves in the Confederacy.
Fortunately for the Union, that didn't really happen.
They were called Copperheads and McClellan was the ‘64 candidate they backed.